Mass spectrometry (MS) describes a variety of instrumental methods of qualitative and quantitative analysis that enable sample components to be resolved according to their mass-to-charge ratios. For this purpose, a mass spectrometer converts the components of a sample into ions, sorts or separates the ions based on their mass-to-charge ratios, and processes the resulting ion output (for example, ion current, flux, beam, signal, et cetera) as needed to produce a mass spectrum. Typically, a mass spectrum is a series of peaks indicative of the relative abundances of charged components as a function of mass-to-charge ratio. The term “mass-to-charge” is often expressed as m/z or m/e, or simply “mass” given that the charge z or e often has a value of 1. The information represented by the ion output can be encoded as electrical signals through the use of an appropriate transducer to enable data processing by analog and/or digital techniques.
Insofar as the present disclosure is concerned, MS systems are generally known and need not be described in detail. Briefly, a typical MS system generally includes a sample inlet system, an ion source or ionization system, a mass analyzer (also termed a mass sorter or mass separator) or multiple mass analyzers, an ion detector, a signal processor, and readout/display means. Additionally, the MS system typically includes an electronic controller such as a computer or other electronic processor-based device for controlling the functions of one or more components of the MS system, managing data acquisition, storing information produced by the MS system, providing libraries of molecular data useful for analysis, and the like. The electronic controller may include a main computer that includes a terminal, console or the like for enabling interface with an operator of the MS system, as well as one or more modules or units that have dedicated functions such as data acquisition and manipulation. The MS system also includes a vacuum system to enclose the mass analyzer(s) in a controlled, evacuated environment. In addition to the mass analyzer(s), depending on design, all or part of the sample inlet system, ion source, and ion detector may also be enclosed in the evacuated environment.
In operation, the sample inlet system introduces a small amount of sample material to the ion source, which may be integrated with the sample inlet system depending on design. In hyphenated techniques, the sample inlet system may be the output of an analytical separation instrument such as a gas chromatographic (GC) instrument, a liquid chromatographic (LC) instrument, a capillary electrophoresis (CE) instrument, a capillary electrochromatography (CEC) instrument, or the like. The ion source converts components of the sample material into a stream of positive and negative ions. One ion polarity is then accelerated into the mass analyzer. The mass analyzer separates the ions according to their respective mass-to-charge ratios. The mass analyzer produces a flux of ions resolved according to m/z ratio and the ions are collected at the ion detector.
The ion detector functions as a transducer that converts the mass-discriminated ionic information into electrical signals suitable for processing/conditioning by the signal processor, storage in memory, and presentation by the readout/display means. A typical ion detector includes, as a first stage, an ion-to-electron conversion device. Ions from the mass analyzer are focused toward the ion-to-electron conversion device by means of an electrical field and/or electrode structures that serve as ion optics. The electrical and structural ion optics are preferably designed so as to separate the ion beam from any neutral particles and electromagnetic radiation that may also be discharged from the mass analyzer, thereby reducing background noise and increasing the signal-to-noise (S/N) ratio. The ion-conversion stage may be followed by an electron-multiplier stage, which typically includes dynodes for multiplication and an anode for collecting the multiplied flux of electrons and transmitting an output electrical current to subsequent processes. Alternatively, a photomultiplier may be substituted for an electron multiplier and operated in a similar manner.
The output of an ion detector typically is an amplified electrical current proportional to the intensity of the ion current fed to the ion detector and the gain of the electron multiplier. This output current can be processed as needed to yield a mass spectrum that can be displayed or printed by the readout/display means. A trained analyst can then interpret the mass spectrum to obtain information regarding the sample material processed by the MS system.
Examples of ion sources include, but are not limited to, gas-phase ion sources and desorption ion sources. Ion, sources may also be characterized according to whether they implement hard ionization or soft ionization. One example of an ion source is an electron impact ionization (EI) source. In a typical EI source, sample material is introduced into a chamber in the form of a molecular vapor. A heated filament is employed to emit energetic electrons, which are collimated and accelerated as a beam into the chamber under the influence of a voltage potential impressed between the filament and an anode. The path of the beam of sample material into the chamber is typically orthogonal to the path of the electron beam. These paths intersect at a region within the chamber, where ionization of the sample material occurs as a result of the electron beam bombarding the sample material. The primary reaction of the ionization process may be described by the following relation:
M+e−→M*+2e−, where M designates an analyte molecule, e− designates an electron, and M*+ designates the resulting molecular ion. That is, electrons approach a molecule closely enough to cause the molecule to lose an electron by electrostatic repulsion and, consequently, a singly-charged positive ion is formed. A voltage potential is employed to attract the ions formed in the chamber toward an exit aperture, after which the resulting ion beam is accelerated into the mass analyzer.
In the operation of an ion source, a phenomenon of ion beam self-oscillation may occur when the source is operated under high electron emission currents (hundreds of micro-amps) and strong magnetic fields (hundreds of Gauss) in order to maximize its sensitivity. This phenomenon may manifest itself by a quasi-periodic oscillation of the ion signal extracted toward the mass spectrometer, with frequencies that vary according to the conditions of the source over a wide range: from Hz to hundreds of kHz. When this self-oscillation phenomenon occurs, the performance of the mass spectrometer may be degraded, leading to poor peak area reproducibility, poor linearity, and inconsistent ion ratios measured. The phenomenon occurs with higher probability when high electron emission currents are employed, when the source is operated at low pressures (<1 mTorr), and when the ion extracting lens voltage is small (a few volts).
Based on experimental observation of the inventors in the present disclosure in the use of EI sources, the following mechanism for the observed self-oscillation phenomenon is proposed, with the understanding that there is no intention to limit any aspect of the present disclosure by such proposal. Inside the ion source, the electron space charge may create a potential well around the electron beam. The ions that are generated by the electrons may be trapped in this potential well for a finite time before they can be extracted toward the mass spectrometer. Under certain conditions, particularly when the electron density is maximized in order to maximize the sensitivity of the source, the trapped ions may only be able to escape the electron potential well after they accumulate in large number, through charge repulsion, in a burst. This mechanism of ion extracting could lead to the self-oscillation of the ion beam where a cycle consists of a short ion burst followed by a time when the ions are trapped and accumulate around the electron beam.
It is acknowledged, therefore, that a need exists for a solution that would inhibit the occurrence of the ion self-oscillation phenomenon but at the same time would preserve the overall sensitivity of the ion source. Experiments of the inventors in the present disclosure have indicated that the phenomenon of self-oscillation could be prevented by a series of possible actions. First, the self-oscillation could be prevented by reducing the electron density in the ion source, either through reducing the electron filament current or reducing the strength of the electron-collimating magnetic field. Unfortunately, for a given geometry of the ion source, this leads to a significant reduction of the overall sensitivity of the source. Second, the self-oscillation could be prevented by increasing the voltage applied on the first ion extracting lens, but this again is limited by a given source geometry, so it could lead to a significant decrease of the sensitivity of the source. Third, the self-oscillation could be prevented by increasing the background gas present in the source that usually is the carrier gas of the gas chromatograph, for example, helium. This pressure has a limited range of adjustment due to the specific flow rates needed to operate the gas chromatograph, so it is not presently deemed to be an acceptable solution either.
Accordingly, there continues to be a need for providing an adequate solution for controlling space charge-driven ion instabilities in ion sources, and particularly EI ion sources.